In automatic shifting power transmissions, gear ratio changes are effected by selectively connecting members of planetary gear sets. This is accomplished by selectively engaging torque-transmitting devices such as brakes or clutches. For each gear ratio change, there is a corresponding sequence of off-going clutch disengagement and on-coming clutch engagement. By providing a regulated fluid pressure gain to the clutches, the smooth engagement and disengagement of the clutches may be accomplished.
Additionally, a majority of automatic shifting power transmissions employ a hydrodynamic fluid drive, such as a torque converter or a fluid coupling, between the power source (engine) and a multi-speed gear configuration, such as the planetary gear arrangement discussed above. This hydrodynamic fluid drive will allow the vehicle to come to rest without stalling the engine and will provide a measure of isolation preventing the torsional vibrations, caused by the firing events of the engine, from being transmitted though the powertrain.
A torque converter is a specialized form of fluid coupling that provides a torque multiplier between the engine and gearing to improve the launch performance of the vehicle. The torque ratio of the torque converter is generally in the range of 1.6 to 3.3 depending on the particular application.
As is well known, the hydrodynamic fluid drive is a slipping drive that has a high efficiency loss at vehicle launch. This loss decreases, but is still present, as the hydrodynamic fluid drive approaches a 1 to 1 speed ratio at high speed and low torque. A lock-up clutch is typically added to most automatic shifting power transmissions to effectively remove the hydrodynamic fluid drive from the power path and thereby improve the overall efficiency of the transmission.
The stall speed of the hydrodynamic fluid drive is an important consideration for proper vehicle launch. The stall speed is the speed at which the hydrodynamic fluid drive will hold the engine speed and not allow further gain. The stall speed is usually chosen based on engine torque characteristic, vehicle weight, vehicle duty cycle, etc. A properly selected stall speed will allow the engine to spin to the peak torque range to affect a strong vehicle launch. The stall speed of the converter cannot be varied or tuned while in use, therefore, there may be certain trade-offs such as part throttle efficiency when selecting the stall speed.
Other considerations have been given to improving the overall efficiency of the transmission. For example, the use of a starting clutch in lieu of a torque converter has been suggested and in some instances utilized. The advent of electronic controls improves the operation of a starting clutch as a vehicle launch device. The clutch is, however, a rotating device with all of the complexities associated with such a device. The control needs considerable accuracy to insure consistent fill times and to compensate for variable fluid leaks at the rotating shaft seals. This requires accurate hydraulic flow volumes and pressure control over a wide range of operating requirements. The starting clutch fulfills the requirements of the hydrodynamic fluid drive. These include, for example, shift and launch quality, driveline isolation, mass, peak acceleration, and durability.